The invention relates to RF and microwave filters. In particular, the invention relates to tuning coupled resonator filters used at RF and microwave frequencies.
Filters are important to the performance and operational viability of many modern radar, communcations and telecommunications systems. In most applications, filters provide selective control of frequency bands or extents of transmitted and/or received signals. Among the wide variety of filter types, coupled resonator filters are perhaps the most widely used, especially for applications requiring a bandpass frequency response characteristic in the radio frequency (RF) and/or microwave fequency ranges. Key featurs of many of the well-known coupled resonator filter realizations are the ability of such filters to provide relatively low passband loss and very high stopband isolation. In fact, for systems having operating frequencies in the RF and/or microwave range, a coupled resonator filter is often the only filter type that can provide adequate performance. Well-known coupled resonator filter realizations for RF and microwave applications include, but are not limited to, various microstrip and stripline filters, such as so-called combline filters, half wavelength couple line filters, hairpin line filters and interdigital filters; and waveguid filters, such as coupled cavity filters.
As the name implies, the coupled resonator filter comprises a plurality of resonant structures or resonators located between an input and an output of the filter. Each resonator has a frequency of resonance called a resonator frequency. Furthermore, each resonator is coupled to one or more of the other resonators in the filter. A given frequency response of the coupled resonator filter is produced by carefully controlling a relationship betweenthe resonator frequencies and coupling factors between resonators. Through such careful control, a diverse assortment of amplitude vs. frequency and/or phase vs. frequency characteristics both inside and outside of the filter passband can be achieved.
Although coupled resonator filters have a variety of designs and realizations, most of these filters provide for at least some form of tuning. The tuning enables adjustment of the filter following or during manufacture to account for variations in manufacturing and device tolerances. For example, a waveguide coupled-cavity filter is typically provided with a plurality of tuning screws that facilitate adjustment of the resonator frequencies and, in some cases, the coupling factors between resonators. During the manufacture of a cavity filter, the filter is tuned to match a design specification for the filter by changing the position of the various tuning screws. In some cases, tuning is used to expand the applicability of a given filter design. Center frequency and passband/stopband performance characteristics can often be adjusted by tuning to meet more than one particular performance specification with a single filter design. Thus, effective tuning can both reduce manufacturing costs associated with tolerance control and increase the applicability of a given filter design by enabling the filter to be adjusted according to a specification for particular application.
Unfortunately, tuning a coupled resonator filter can be and generally is very difficult. In particular, tuning coupled resonator filters is affected by a phenomenon known as frequency pushing. Frequency pushing is a tendency of a resonator frequency of a given resonator to be altered by a change in a resonator frequency of another resonator. For example, during tuning, a resonator frequency of a first resonator may be adjusted followed by an adjustment of a resonator frequency of a second resonator. If the resonator frequency of the first resonator is then checked, it is often the case that the resonator frequency of the first resonator will have changed as a result of inter-resonator interaction between the first and second resonator. In practice, the frequency pushing due to inter-resonator interaction is the result of coupling between resonators, a situation that generally can""t be avoided in the coupled resonator filter. The usual approach is to iteratively tune each of the resonators multiple times until a specified tuned filter response for the filter is achieved. This iterative tuning can be very time consuming and therefore is relatively costly. In addition, a highly trained technician is typically required to perform the tuning. The need for iterative tuning by a skilled technician greatly increases the cost of most coupled resonator filters.
Accordingly, it would be advantageous to have an approach to tuning coupled resonator bandpass filters that accounted or compensated for frequency pushing due to inter-resonator interactions. Moreover, it would be advantageous if such a tuning approach for coupled resonator bandpass filters were deterministic and largely non-iterative. Such a tuning approach would address a longstanding need in the area of coupled resonator filter manufacturing and tuning.
The present invention provides for tuning coupled resonator filters. In particular, the present invention provides an essentially non-iterative, deterministic tuning of resonators of a coupled resonator filter. According to the present invention, frequncy-pushing interations between resonators of the filter are determined and the frequency of each resonator of the filter is adjusted. The adjustment accounts for the deteimined frequency-pushing ineractions. Moreover, a frequency change is determined for each resonator of the filter to be tuned, such that when the frequency change is applied to the resonators, the filter will be properly tuned with resect to a given tuning specification.
In essence, the present invention determines a nature and an amount of interaction between the resonators and then adjusts the tuning of the filter in an appropriate manner based on the determination. Furthermore, since the determined frequency change accounts for frequency-pushing interactions between resonators, tuning the filter according to the present invention can often be accomplished without the need for multiple iterations. The present invention is well suited for manual, semi-automatic and even automatic tuning of coupled resonator filters.
In one aspect of the present invention, a method of tuning a coupled resonator filter is provided. The method of tuning comprises determining a resonator frequency for each resonator of a reference or xe2x80x98goldenxe2x80x99 filter. The resonator frequencies of the esonators of the reference filter are preferably determined by direct measurement of the reference filter. Alternatively, the resonator frequencies for the reference filter may be extracted from a model of the reference filter.
The method of tuning further comprises determining frequency pushing effects or parameters for each of the resonators with respect to the other of the resonators. The determination may be performed either using the reference filter or the subject filter, but preferably uses the reference filter. Determining frequency-pushing parameters comprises deriving a functional relationship between a change in a resonator frequency of one resonator due to a change in the resonator frequency of another resonator. Functional relationships are determined for all combinations of resonators. In a prefered embodiment, the functional relatonships are linear functions and a frequency-pushing matrix is assembled that contains slopes of the linear functions.
The method further comprises measuring a frequency of each resonator of the subject filter. As was the case for the reference filter, measuring a frequency of each resonator of the subject filter is preferably accomplished by direct measurement of the subject filter.
The method further comprises computing a frequency change for each resonator of the subject filter. In a preferred embodiment, computing the frequency change comprises inverting the frequency-pushing matrix and computing a vector difference between a first or reference frequency. vector containing the determined reference filter resonator frequencies and a second vector containing the measured subject filter resonator frequencies. Computing the frequency change further comprises multiplying the difference by the inverted frequency-pushing matrix thus yielding a vector containing frequency changes, one for each resonator.
The method further comprises applying the computed frequency changes to the resonators of the subject filter. When applied to the resonators, the frequency changes tune the subject filter, such that the performance of the tuned subject filter closely matches that of the reference filter.
In another aspect of the invention, a tuning system for tuning a coupled resonator filter is provided. The tuning system comprises a tuning controller, a memory, a tuning program stored in the memory, and a measurement system. The controller, by executing the computer program, controls the measurement system to produce measurements of a subject filter being tuned and a reference filter that serves as a gauge for the tuning of the subject filter. Instructions of the computer program preferably implement the method of tuning according to the present invention. In various embodiments, the tuning system can be a manual tuning system, a semi-automatic tuning system, or an automatic tuning system.
In still another embodiment, a tuned coupled resonator filter is provided. The tuned filter comprises a plurality of resonators. The plurality of resonators is tuned by the method of the present invention using a reference filter.
Advantageously, the present invention provides a simple, scalable, deterministic procedure for tuning of bandpass coupled resonator filters of various complexities. Further, the deterministic nature of the tuning of the present invention essentially eliminates the need for iterative tuning. In general, tuning may be accomplished in a single iteration of determining frequency changes of the resonators of the filter. The present invention is particularly attractive for manufacturing environments where significant production cost savings can be realized through reduced tuning time and difficulty. Moreover, the present invention is ideally suited for both semi-automatic and automatic filter tuning systems. In particular, the present invention reduces the need for such a skilled technician, as was conventionally necessary, in the tuning of production filters once the reference filter is available.
Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings.